Metallocene catalyst systems are extensively used in a variety of polymerisation systems, including the polymerisation of olefins. Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane activating agent, such as methylaluminoxane (MAO). This resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerisation media. These homogenous catalyst systems have the disadvantage that when they are used under slurry polymerisation conditions, they produce polymers which stick to the reactor walls during the polymerisation process (generally referred to as “fouling”) and/or polymers having small particle size and low bulk density which limit their commercial utility.
Various methods have been proposed in an effort to overcome the disadvantages of the homogenous metallocene catalyst systems. Typically, these procedures have involved the prepolymerisation of the metallocene aluminoxane catalyst system and/or supporting the catalyst system components on a porous carrier (also known as a “particulate solid” or “support”). The porous carrier is usually a silica-containing support.
Another important consideration in the development of metallocene catalysts is the yield of solid polymer that is obtained by employing a given quantity of catalyst in a given amount of time. This is known as the “activity” of the catalyst. There is an ongoing search for metallocene catalysts and techniques for preparing such catalysts which give improved activity for the polymerisation of olefins. An improved activity means that less catalyst needs to be used to polymerise more olefins, thereby reducing the costs considerably, since metallocenes are more expensive than Ziegler-Natta and chromium catalysts.
Several attempts have been made to titanate silica supports for use in metallocene catalysed ethylene polymerisations. Jongsomjit et al. (Molecules 2005, 10, 672, Ind. Eng. Chem. Res. 2005, 44, 9059 and Catalysis Letters Vol. 100, Nos. 3-4, April 2005) discloses the titanation of silicas for zirconocene catalysed ethylene polymerisation, wherein the support is prepared according to Conway et al. (J. Chem. Soc., Faraday Trans. J, 1989, 85(1), 71-78) using mixed supports of titania and silica mixed-oxide supports. The increase in activity with such a support is only of 25%, because the titania is present predominantly in its anatase form i.e. a crystalline form. Under polymerisation conditions, little morphological control can be obtained with such a support. It is particularly difficult to use industrially, since the porous volume, bulk density and particle size of both the silica and titania need to be similar in order to avoid decantation of one with respect to the other. In addition, the interaction of the Ti with the actives sites is not optimized.
Thus, a new catalyst support is needed for metallocene catalysts which can induce improved activity of the metallocene catalyst system, particularly under industrial conditions.
An object of the present invention is to provide a new catalyst support for metallocene catalysts to increase their activity.
Furthermore, it is an object of the present invention to provide a new method for polymerising olefins, preferably ethylene and propylene, using a new supported metallocene catalyst system.